The present invention relates to an inexpensive FE--Ni--Cr-base superalloy which is excellent in high-temperature strength, and an engine valve for a motor vehicle and a knitted mesh supporter for an exhaust gas catalyzer for a motor vehicle which are made of the alloy.
In recent years, there has been demand for saving energy and treating exhaust gas to solve the global problems of environmental pollution. It has also been desired to reduce the cost of material for manufacturing-component parts. In order to achieve such objects, there has been a strong demand for reducing the cost of high-quality materials, such as an engine valve material and an exhaust gas mesh material, which are exposed to the highest temperature and the highest stress in an internal combustion engine of a motor vehicle or the like.
Conventionally, as a material for exhaust gas valves of a gasoline engine or a diesel engine, JIS SUH35 (Fe-8.5Mn-21Cr-4Ni-0.5C-0.4N) which is a high-Mn austenitic steel has been widely used. However, in accordance with an increase of the use temperature, JIS NCF751 (Ni-15.5Cr-1Nb-2.3Ti-1.2Al-7Fe), which is an Ni-base superalloy, has started to be employed in some cases. However, JIS NCF751, which contains about 70% Ni, is by far more expensive than JIS SUH35. Therefore, there have been developed alloys which contain less costly alloying elements than JIS NCF751. These less costly alloys are designed to have properties, such as high-temperature strength and stability of alloy structure after long-time heating, which are as close to those of JIS NCF751 as possible.
As a result, there have been made many suggestions such as JP-B2-1-12827, JP-A-62-214149, JP-A-58-189359, JP-A-63-213631, JP-A-61-238942, JP-B2-62-50542, JP-B2-4-11613, JP-A-60-211028 and so on.
As gasoline fuel for engines of motor vehicles, lead-free gasoline has recently been employed for answering the demand for treatment of exhaust gas, and engines for which lead-free gasoline is exclusively used have been mainly produced. For the engine parts which are used at a particularly high temperature, such as engine valves and knitted mesh supporters for exhaust gas catalyzers for a motor vehicle, use of lead-free gasoline leads to an improvement of corrosion environment. If the same level of oxidation resistance property as JIS NCF751 is provided, no consideration need be given to the corrosion resistance property with respect to lead oxide, which has conventionally been a problem to be considered.
On the other hand, in accordance with prolonged guarantee periods for motor vehicles, there has been a need to improve the performance of alloys, particularly in terms of the durability thereof. An alloy, whose strength deterioration and embrittlement after high-temperature long-time use are as small as possible is now in demand.
Of the cost-saving replacements for JIS NCF751, which replacements contain less costly alloying elements, the alloys proposed in JP-A-63-213631, JP-B-2-4-11613 and JP-A-60-211028 can provide high-temperature strength and long-time stability of alloy structure which are close to those of JIS NCF751. However, since the Ni content exceeds 50%, reduction of costs is not sufficiently accomplished as compared with JIS NCF751. The alloys proposed in JP-B2-1-12827, JP-A-62-214149 and JP-A-58-189359 have excellent oxidation resistance and corrosion resistance properties, because Cr content is high. However, unfavorable phases which deteriorate normal-temperature ductility, such as the .sigma. phase and the .alpha.' phase rich in Cr precipitate, are present. The alloys proposed in JP-A-61-238942 and JP-B2-62-50542 have low-Ni and low-Al compositions. Consequently, during long-time heating, coarsening of the .gamma.' (gamma prime) phase, which is a precipitation strengthening phase, and transformation from the .gamma.' phase into the .eta. (eta) phase occur, thereby increasing the deterioration of high-temperature strength after long-time heating.